PHOTOREACTIVATION OF KILLING IN STREPTOMYCES‐II. KINETICS OF FORMATION AND PHOTOLYSIS OF PYRIMIDINE DIMERS AND ADDUCTS IN S. COELICOLOR AND S. GRISEUS PHR‐1*

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Abstract
Abstract— A pyrimidine adduct, 6‐4‘‐[pyrimidine‐2’‐one] thymine (PO‐T)‡, observed in DNA hydrolysates of 254‐nm ultraviolet (u.v.) irradiated conidia of Streptomyces coelicolor, increases linearly with u.v. dose up to 2 × 105 ergs/mm2. Yields of thymine dimer (T○) and uracil‐thymine dimer (U○) level off at much lower doses. Initial relative rates of formation of these u.v. photoproducts are: 1:1.3:4.8 for PO‐T, T○ and U○, respectively. Similar results were obtained with a Streptomyces griseus mutant, PHR‐1.An equation is derived to estimate the ratio of the amount of PO‐T to the total amount of thymine‐derived photoproducts at low (biological) u.v. doses. The observed PO‐T fractions compare well with the calculated values.Rapid photolysis of the precursor of PO‐T was observed by post‐u. v. treatment at 313 nm of conidia of S. coelicolor and of S. griseus PHR‐1. The photolysis was much slower at 365 nm and did not occur at all at 405 nm. Pyrimidine dimers were not appreciably affected by post‐u. v. treatment at the above wavelengths in these Streptomyces strains. Both of these strains are phenotypically photoreactivation‐deficient, and the present results indicate that they do not possess active photoreactivating enzyme.In earlier papers[3,4,5], the pyrimidine adduct found in acid hydrolysates of DNA was loosely referred to as “uracil‐thymine adduct (U‐T adduct)”. Such terminology is not strictly correct. The pyrimidine adduct in acid hydrolysates is PO‐T (sometimes called P2B), which could theoretically result from removal of ammonia from a C‐T adduct or removal of water from a U‐T adduct (see [6]).